The present invention relates to a fuse terminal (which will be also called a fusible link) for protecting a wire harness, and more particularly to a fuse terminal having an improved structure capable of attaining satisfactory fusing characteristics with use of a Cu alloy having a good spring performance and a conductivity of 20--less than 60% IACS.
Generally, the fusible link is fusible in a short time if an excess current flows in a circuit, thereby preventing damage to the wire harness and associated electrical equipment and also preventing a secondary accident such as burning. From the viewpoint of ease of handling, the fusible link usually comprises a fuse terminal having a fusible conductor with a fuse portion and connection terminals integrally formed with the fusible conductor.
An ideal fusing characteristic of the fusible link is shown in FIG. 4, using a motor of a 15 A rating current and a connection wire of AVS 1.25 sq (mm.sup.2), for example.
A short-circuit is generally classified as a slight short-circuit or a dead short-circuit.
The dead short-circuit in case of AVS 1.25 is shown by a current waveform denoted by A in FIG. 4. Namely, the dead short-circuit is a short-circuit of such a kind where a relatively large current flows in the circuit. In designing an automotive wire harness, it can be generally sufficiently protected if the fusible link is fused within five seconds in case of such a large current. Drawing a dotted line Q corresponding to five seconds in parallel relation with an X-axis (current) and plotting an intersection P of the dotted line Q and the curved line A, a fusing characteristic curve of the fusible link must pass on the left side of the intersection P. However, a large current is instantaneously generated as shown by a curved line B upon starting of the motor. If the fusing characteristic curve is overlapped with the curved line B, the fusible link cannot effectively function. Accordingly, the fusing characteristic curve for the dead short-circuit must pass in a hatched region S, and especially from the viewpoint of durability, it must pass near the curved line A far away from the curved line B so as to lengthen the life of the harness.
On the other hand, the slight short-circuit is a short-circuit of relatively small current as generated in a region surrounded by T shown in FIG. 4. A curved line C denotes a smoking characteristic curve of AVS 1.25. If the fusing characteristic curve is overlapped with the curved line C, the fusible link cannot effectively function. Accordingly, the fusing characteristic curve of the fusible link for the slight short-circuit must pass in a cross hatched region S' where the fusing characteristic curve is not overlapped with the curved line C. However, if the fusing characteristic curve passes near the line of the rating current of 15 A, heat generation of the fusible link is increased upon supplying of the normal current of 15 A. Therefore, the fusing characteristic curve preferably passes near the curved line C far away from the line of 15 A.
For the above reasons, the ideal fusing characteristic is shown by a curved line D in FIG. 5.
As to the relation between the fusing characteristic and a material of the fusible conductor, it has been found that the fusing characteristic is classified into three kinds of curves E, F and G as shown in FIG. 6 in dependence upon a conductivity of the material. The relation between the curves and the conductivity is as follows:
______________________________________ Curve Conductivity (%) ______________________________________ E 60 and more F 20-less than 60 G less than 20 ______________________________________
The difference in the fusing characteristic curves E-G in dependence upon the conductivity is due to the following reasons. As a fusing time is instantaneous in case of a large current, the fusing characteristic depends on resistance, and temperature increase is rapid to reduce heat radiation performance. Accordingly, the samples of the fusible link are prepared to have the same resistance of the material, so as to make the dead short-circuit characteristics identical to one another. Therefore, the fusible link is made narrow in case of a high conductivity, while it is made wide in case of a low conductivity. This structure influences upon the heat radiation performance at the generation of the slight short-circuit. That is, temperature increase is gentle because of a small current, and the heat radiation characteristic largely influences upon the fusing characteristic. The lower is the conductivity, the greater is the width (surface area) of the material, enhancing the heat radiation performance, but reducing the fusing characteristic.
As is mentioned above, the fusible link having the ideal fusing characteristic may be produced by using a material having a conductivity of 60% and more. However, since such a material is inferior in spring performance, a fused terminal using the material as a spring material and having connection terminals formed integrally therewith is less reliable in electrical connection with a mating terminal. For example, a commercially available spring material has usually a conductivity of 30% or less. Although a spring material having a conductivity of about 50% and a good spring performance has been recently developed owing to an advanced technology, a conductivity of 60% has not yet been reached.
In a conventional fuse terminal as disclosed in Japanese Utility Model Publication No. 60-6988, a fusible conductor and a spring member (terminal portion) are formed of different materials. However, it is preferable to produce a fused terminal as an integral part from the viewpoint of a manufacturing cost. Further, a material having a conductivity of 65% has been proposed in Japanese Patent Laid-Open Publication No. 58-163127. However, unless the fusible conductor has a sufficient length, desired fusing characteristics cannot be obtained because of too high conductivity. As disclosed in Japanese Utility Model Publication No. 59-41563, it is required to make the fusible conductor (fuse portion) sufficiently long and seal a portion of the fusible conductor except the fuse portion by means of a heat absorbing member of an inorganic material so as to make up a lack of strength of the product. However, owing to a heat radiation effect of the heat absorbing member, a fusing characteristic is shown by the curved line F in FIG. 6. Further, the material disclosed in Japanese Patent Laid-Open Publication No. 58-163127 is known as a CDA 194 alloy having a spring limit value of about 23 kg f/mm.sup.2, which does not satisfy a required value of 40 kg f/mm.sup.2 for the spring member.
If the curved line F is intended to satisfy the slight short-circuit by increasing a resistance, it merely shifts to a curved line F', and is overlapped with the instantaneous current waveform of the motor (at a point P'). Accordingly, durability of the fuse terminal is reduced to result in no utility.